1. Field of the Invention
The present invention relates to an optical system, and more particularly, to a digital light processing (DLP) optical system for solving a contrast problem of an image that is caused by light discarded in the DLP optical system.
2. Description of the Related Art
A DLP optical system is much favored as a panel for a next generation projection television (TV) and a projector because the DLP optical system has a superior contrast to a contrast of a panel of liquid crystal display (LCD) devices, cathode ray tubes (CRTs), which are currently in use, and has a very simple optical structure, so that a size and weight of the system can be reduced.
The DLP optical system allows light from a lamp to be reflected by a plurality of extremely fine mirrors manufactured using a Micro Electro Mechanical System (MEMS) technology and controls brightness of a pixel corresponding to each of the mirrors. At this point, since light's brightness expresses nothing but a contrast, light is transmitted through a color wheel before the light is incident to the mirrors so that both contrast and color can be realized.
The brightness of each pixel is determined by a time for which one of the mirrors that corresponds to each pixel illuminates light from a light source onto a projection lens. Such a process is realized through pulse width modulation (PWM).
The DLP optical system uses a digital micro mirror device (DMD) developed by Texas instrument Co. The DMD is a semiconductor light switch where fine driving mirrors are integrated. In the DMD, a fine aluminum alloy mirror is installed on an upper portion of one cell of a static random access memory (SRAM), and the fine mirror is operated by a static electric field of a lower memory.
The DMD is classified into a single-chip DMD and a 3-chip DMD separating light into red, green, and blue lights, and managing the same. Both the single-chip DMD and the 3-chip DMD are complete digital type devices and have an advantage of achieving very excellent color reproduction. Besides, the DMD has a long life, a high definition, and can directly reproduce an input digital image signal without separate correction and thus realize an image without reduction of image quality.
A construction of a DLP optical system will be schematically illustrated. First, the DLP optical system includes a lamp unit emitting light, an optical engine unit for converting light introduced from the lamp unit into an appropriate image signal on the basis of an image signal input from the outside, an DMD device for reflecting an image signal transmitted from the optical engine unit, a projection lens for transmitting part of the image signal reflected from the DMD device, and a support for supporting respective elements.
The lamp unit is used as a light source for providing white light including all colors and may be a metal halide lamp. The metal halide lamp emits light by discharging electricity in a gas in order to provide brighter complete white light than that of a low-priced metal filament lamp.
The optical engine unit separates and combines colors of whit light generated from the lamp unit such that colors correspond to an image signal input from the outside to form an image and projects the formed image.
The DMD reflects an light in the form of an on-beam or an off-beam according to a signal input from the outside. The one-beam or off-beam is changed in its path by a prism and projected to the outside via the projection lens.
In detail, when needed, the DMD reflects an input signal in the form of an on-beam or an off-beam. When the input signal is reflected in the form of the on-beam, the DMD realizes a white screen. When the input signal is reflected in the form of the off-beam, the DMD realizes a black screen. There can exist a reflection angle of a DMD in an on-beam state, a reflection angle of a DMD in an off-beam state, and an intermediate angle between them. This is because the DMD realizes the on-beam state and the off-beam state by physically rotating mirrors of the DMD. Therefore, a flat state is generated when transition from the on-beam state to the off-beam state, and from the off-state to the on-state are made.
A light optical path of the DLP optical system having the above-descried construction is clearly understood when a conceptual view of a light reflection path of the DMD illustrated in FIG. 1.
Referring to FIG. 1, when mirrors 8 of the DMD rotates a predetermined angle, light emitted from a lamp 5 is incident to a projection lens 6 when the light is in the on-beam state and incident to a light absorption part 7 when the light is in the off-beam state. However, in the intermediate state between the on-beam state and the off-state, light is incident to a portion not defined in an inner side of the optical system or incident to a screen via the projection lens, so that an unwanted image is formed on the screen.
The intermediate state between the on-beam state and the off-beam state will be descried in detail.
When the DMD transitions from the on-beam state to the off-beam state or vice versa, the mirrors of the DMD that reflect light go through an intermediate angle while the mirrors change between an angle corresponding to the on-beam state and an angle corresponding to the off-beam state. Therefore, even during the off-beam state, part of light is transmitted via a prism and projected onto a screen via the projection lens by the mirrors. The light projected onto the screen during the off-beam state is illuminated on an entire screen, which reduces contrast of the screen. Accordingly, black color is not perfectly realized but instead a gray color is displayed.
In detail, even when a related art DLP optical system realizes a black screen, that is, the DMD reflects all signals in the form of the off-beam, part of light is projected onto a screen via a projection lens, so that a perfect black and white screen is not realized and thus contrast of the screen is reduced.
Since such contrast reduction is always generated while mirrors of a DMD rotate, the contrast reduction is always generated when an image of any type is formed on a screen as well as a black is realized on the screen.